void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    // vec2 p = fragCoord / iResolution.xy-vec2(0.5,0.5);
    //介绍fragCoord和iResolution的关系
  vec2 p = 2.0 * (fragCoord-0.5*iResolution.xy)/iResolution.xx;
    float length1 = length(p);
    vec3 col;
    if(length1 < 0.2) {
        col = vec3(1.0, 1.0, 1.0);
    } else {
        col = vec3(0.0, 0.0, 0.0);
    }
 
    fragColor = vec4(vec3(col), 1.0);
}